1. The Field of the Invention
Embodiments of the present invention generally relate to vertical cavity surface emitting lasers (VCSELs) and optical transceivers incorporating such VCSELs. More particularly, embodiments of the present invention relate to VCSELs having improved active regions.
2. Related Technology
Solid-state semiconductor lasers are important devices in applications such as optoelectronic communication systems and high-speed printing systems. Among the different types of lasers, there has been an increased interest in vertical cavity surface emitting lasers (VCSELs). One reason for growing interest in VCSELs is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. Furthermore, edge emitting lasers cannot be tested until the wafer is cleaved into individual devices, the edges of which form the mirror facets of each device. By contrast, not only does the beam of a VCSEL have a small angular divergence, a VCSEL emits light normal to the surface of the wafer. Additionally, because VCSELs generally incorporate mirrors monolithically in their design, they allow for on-wafer testing and the fabrication of one-dimensional or two-dimensional laser arrays.
VCSELs are typically made by growing several layers on a substrate material. VCSELs include a first mirrored stack, formed on the substrate by semiconductor manufacturing techniques, an active region, formed on top of the first mirrored stack, and a second mirrored stack formed on top of the active region. By providing a first contact on top of the second mirrored stack, and a second contact on the backside of the substrate, a current is forced through the active region. Currents through the VCSEL above a threshold current cause laser emissions from the active layer as electrons switch from the conduction band to the valance band, producing photons and thereby generating the light.
The active region is further made up of one or more quantum wells sandwiched between two spacer cladding regions. Inside the spacers, the active region is sandwiched by confining layers. The confining layers or regions are used to provide electrical confinement of minority carriers. By selecting the appropriate materials for the quantum well, the confining layers, and the barrier layers, a VCSEL generally may be grown or fabricated that generates light at a desirable, predetermined wavelength. For example, by using InGaAs quantum wells on GaAs substrates, longer wavelength VCSELs can be produced. The use of InGaAs quantum wells, however, causes strain in the quantum wells. If the quantum wells are grown past their critical thickness, they can relax by creating dislocations, and thus a degraded active region results.
VCSELs made with GaAs and that emit light in the 850 nanometer range are known in the art. Because the quantum well for the short wavelength 850 nanometer VCSELs is made from GaAs (the same material as the substrate) the various epitaxially deposited layers, whose thickness is related to wavelength, are able to maintain the minimal mechanical strain without mechanical relaxation. If one were to use InGaAs in the active region at the larger 1.3 μm wavelength device range (e.g., 1200-1650 nm), however, the lattice mismatch is generally such that the quantum well layers would tend to relax their strains and suffer dislocations, produce slip lines or develop island growth, which would interfere with proper lasing by acting as non radiative recombination centers.
In order to go to the proper bandgap for what is referred to in the art as a 1.3 μm wavelength (i.e. greater than 1260 nm) semiconductor lasers, one generally uses InGaAs, GaAsSb or some combination thereof instead of GaAs in the active layer. Indium gallium arsenide (InGaAs) and gallium arsenide antimonide (GaAsSb), however, do not possess the same lattice constant as GaAs at the compositions useful for 1.3 micron lasers. This makes it very difficult to build a proper quantum well structure.
The thickness of the various layers in the active region, while not arbitrary, has some flexibility within the constraints of the design and the process. The combined thickness of the spacers, the confining layers, and the layers of the active regions sandwiched by the mirrors must be such that a Fabry-Perot resonator is formed. The quantum wells should generally be positioned so that they are roughly centered at an antinode of the optical electric field. These two requirements define the spacer thickness in terms of the other layer thicknesses.
Long wavelength quantum wells are a challenge to construct. The semiconductor laser, e.g., VCSEL, art needs means to achieve long wavelength quantum wells normally fabricated on GaAs substrates. It is therefore very desirable to come up with a quantum well (i.e. the active layer (or quantum well layer) and the barrier layers surrounding the active layer) making use of materials such as GaAs, InGaAs or GaAsSb in the construction of a VCSEL operational above the 1200 nm range.
The present inventors recognized that it would be advantageous to remedy the foregoing and other deficiencies in conventional devices and to facilitate the production of longer wavelength VCSELs by introducing Migration Enhanced Epitaxy (MEE) into the VCSEL fabrication process. Additionally, there is a continuing need for improved VCSELS long wavelength VCSELs, and methods of forming the long wavelength VCSELs through improved techniques that use Migration Enhanced Epitaxy (MEE).